In the preparation of propellants it is critically important to properly handle the ingredients of the propellants to assure safety. For many solid rocket propellants, mixers ranging in size from small (one pint) to extremely large (600 to 1000 gallons) are used to combine the ingredients. Such mixers use blades that have narrow tolerance between the blades and the interior wall of the mixer (usually fractions of an inch). During mixing, the ingredients are viscous and later solidify during curing of the mixture. As can be easily appreciated, a major safety concern is to eliminate or reduce undesired friction in the mixture during mixing. Hence, it is imperative that the contents of the mixture contain particles that are finer than the blade clearance. Devastating explosions have occurred as the result of friction caused by mixture contamination by some object or particle that is larger than the blade clearance.
In the manufacture of certain propellants it is particularly important to use ingredients that have a very high surface area to volume ratio. Certain desirable characteristics of a propellant such as burn rates and viscosity are affected by the ratio of surface area to volume of the propellant constituents. Consequently, low density or sub-micron particles of various materials are used frequently in the manufacture of solid propellants.
However, the use of low density or sub-micron particles as ingredients in a propellent mixture can cause significant safety problems. Small, high surface area particles are subject to viscose (stokes) drag in air and are not heavy enough to pass through a screen readily when driven by gravity or vibration. Such materials are too light to fall through a safety screen without an external driving force. For example, low density fumed silica particles (a silicon dioxide product known as "Cab-O-Sil") have a size of approximately 0.03 microns. The sub-micron iron oxide material known as "Pyrocat" has also been used in propellant formulations. This material has a surface area to weight that is of the magnitude of 100 square meters per gram or more. For such materials, the force of gravity is insufficient to draw the material through a #8 mesh safety screen (a screen with mesh openings of about 0.15 inches). Consequently, it is extremely difficult to screen the material for contamination.
Heretofore, there have been two methods for contamination removal from low density materials such as Cab-O-Sil and Pyrocat. One method is to use expensive solid particle fluidizing systems to move the materials through a screen using an air column. This method requires expensive capital investment for the fluidizing equipment, and the equipment is large and bulky and certainly not portable. Additionally, such equipment is usually designed to handle a specific material and not other low density materials.
The other method is simply to sift the material by hand through a mesh screen. As can be appreciated, this method has several drawbacks. The sifting must be done in an enclosed area and the person doing the sifting must wear appropriate breathing apparatus and goggles. Since the materials are so light, if they become airborne it may take minutes or hours for the materials to settle to the ground. Further, some low density materials can be catalytic, thereby introducing a serious hazard for explosion if an inadvertent spark occurs in a room of airborne particles. Also, so little material passes through the screen that the process is extremely time consuming. For example, it has required as much as one half hour to screen 0.4 pounds of Pyrocat for a propellant mix. This means that it may take as much as 133 man hours to screen enough material for a 600 gallon propellant mix.
Hence, it would be a significant advance in the art to provide an apparatus that could quickly separate light, low density, high surface area materials from coarser materials or hazardous contamination where fluidizing systems are not practical or have not been developed or where hand sifting is not practical.